Commercial cellular communications systems and personal communications systems often employ known spread spectrum, modulation techniques for multiple access or for increasing the immunity to noise and interference. Spread spectrum techniques make use of a sequential noise-like signal structure with a wide-band of frequencies, for example pseudonoise (PN) codes, to spread the normally narrow-band information signal over the relatively wide-band of frequencies. The receiver by correlating the received spread spectrum signal with the known PN code wave-form, despreads the signal to retrieve the original information signal. Another characteristic of these systems in a multi-user application is the use of a base station to effectively relay communications from user to user. However, the use of a base station is impractical in forward area battlefield communications. There are currently no commercial cellular communications systems working without a base station which is needed to implement the entire billing and accounting process. Without a base station the operator cannot perform effective billing of the conversations carried on through the system.
Base stations in commercial cellular systems are carefully placed in locations where their antennas, usually positioned very high, on the order of ten to thirty feet, can receive and transmit within the base station area or "cell". In the same way, path loss and multi-path effects are measured such that the base station can determine, through elaborate software control, the best allocation of calls to different frequencies within one area or cell. Also, the commercial base station transmits simultaneously through all the channels available to it to all mobiles in its areas. This is usually forty eight (48) channels for cellular systems and twenty four (24) channels for personal communication systems, which transmit anywhere between twenty (20) Watts and fifty (50) Watts. Moreover, Commercial cellular base stations use a control channel, which is a separate frequency in most systems, to continuously monitor the status and transmitted power of all the telephones which are turned on in the cell.
The use of a base station in commercial cellular systems as discussed above is inappropriate for forward area battlefield communications for several important reasons. Under battlefield conditions, equipment cannot be reliably provided nor can access be gained to all desired sites for setting up fixed base stations. A fixed base station radiating watts of power is undesirable and, the use of a separate control channel easily identifiable and capable of being jammed or taken out by an adversary, compromises the security of such a communications system under battlefield conditions. Lastly, use of a fixed or transportable base stations restricts the soldier's communications and associated operations to areas that are covered by the fixed or transportable base stations.
Accordingly, it is an object of the present invention to disclose a method for personal communications in a code division multiple access (CDMA) system operating without a base station.